The human ankle is crucial to mobility as it counteracts the forces and moments created during walking. Currently there are nearly 2 million people living with limb loss in the United States. Many of these individuals are either transtibial (below knee) or transfemoral (above knee) amputees and require an ankle-foot prosthesis for basic mobility. While there are an abundance of options available for individuals who require an ankle-foot prosthesis, these options fail to mimic an intact ankle when it comes to key evaluation criteria such as range of motion, push-off force, and roll over shape.
The simplest type of ankle-foot prosthesis is the conventional non-articulating SACH (Solid Ankle Cushioned Heel) foot shown in FIG. 6. The SACH foot is able to closely resemble the shape of an actual foot and provides the user with some cushioning during movement. However, it is unable to provide the range of motion and energy return of an intact ankle. Regardless, many less active amputees prefer the SACH foot because of the greater control it gives the amputee.
Unlike the SACH foot, the dynamic response ankle-foot shown in FIG. 11 stores energy during the beginning of the gait cycle and uses the stored energy to propel the foot forward. Also called ESR for Energy Storing and Returning, the energy storage mechanism of the dynamic response ankle-foot is similar to the role of the Achilles tendon. During gait, the Achilles tendon is stretched and stores potential energy that is released during push-off. The energy storage mechanism in dynamic response feet are typically primarily weight activated. This means the prosthetic will store energy while the individual is standing unlike an able-bodied ankle. Despite this difference, the dynamic response ankle-foot provides some resistance to movement similar to that of an intact ankle.
Dynamic response feet can be further classified as either passive or active (microprocessors). Because the energy produced by the ankle joint during average walking speeds is almost completely self-sustaining with no net external energy loss, there is the potential for a purely mechanical mechanism such as the dynamic response ankle to generate the forward motion necessary for an able-bodied gait. However, for speeds faster than normal walking, passive systems are not capable of fully emulating an intact ankle because a positive net external energy is produced by the ankle. The use of an active ankle foot prosthesis for faster speeds may be necessary in the future, but current design limitations make this application less than ideal. An active ankle-foot prosthesis can be over twice as heavy as a conventional prosthesis, are expensive, and experience hardware and control issues adjusting to different speeds. Fundamentally, active prosthetic ankle-feet operate using preplanned kinematic trajectories as opposed to the impedance control mechanism of a human ankle. Finally, while still operating as an ESR system, active ankle-foot prostheses are difficult to customize or match biomimetically in size and weight.
The amount of energy stored in the prosthesis is dependent on the stiffness. Increasing the stiffness will increase the propulsion forces, however, it simultaneously decreases the range of motion (ROM) of the ankle. The ankle joint has a ROM from about 45° plantar flexion to 20° dorsiflexion. Forced to make a choice between propulsion forces and range of motion, many ankle-foot prostheses have only been designed for the ROM that is experienced during gait on an even surface, a value of no more than 30°. While this may seem sufficient as the ROM of the ankle remains consistent with changes in speed, a study looking at individuals with limited ankle ROM due to a sprain showed that ankle ROM does impact gait symmetry in regards to step length and step time. Additionally, ankle ROM is important for walking on sloped surfaces as it helps accommodate for movement about different equilibrium positions.
While both the kinematics and kinetics of an intact ankle are important to its functionality, so far it has been impossible for a passive prosthetic ankle-foot to mimic both. There exists a discrepancy between design changes that improve the kinematics and kinetics. The effect of increasing stiffness is an example of this discrepancy. In an able-bodied ankle, the relationship between angle and push-off moment is linear. However, most prostheses are built with a stiff plastic board that resembles a cantilever beam. A rudimentary knowledge of cantilever beams tells us that the linear relationship between deflection and force is restricted to small deflections and much less than the ankle angle experienced by an able-bodied individual. The stiffness of the foot also impacts the location of the ground reaction forces, and therefore the rollover shape as discussed below. Olesnavage and Winter noticed this effect and suggested the use of a rigid constraint to prevent the foot from over-deflecting.
Recent research in active prostheses has been able to demonstrate the effectiveness of applying a torque that is linear with ankle angle in single subject experiments in a lab environment. Caputo and Collins used a Universal Ankle-Foot Prosthesis Emulator that determined the desired torque by a piecewise linear function in 2014. A team at the Robotics and Multibody Mechanics Research Group at the Vrije Universiteit Brussel is making progress in mimicking both kinematics and kinetics in the development of the actuated prosthetic AMP-Foot. Although not explicitly stated, one of the major changes between the AMP-Foot 2.0 tested in 2014 and the AMP-Foot 3.0 in 2016 was a linear relationship between torque and ankle during initial contact to flat foot. The change resulted in a curve that better mimics an intact ankle as provided by Winter's data and an extra 5 Joules of energy storage. It is interesting to note that the strategy used in the design of active prosthetics to achieve both push-off and range of motion in fast walking speeds is to effectively increase stiffness with ankle angle. While this strategy has been applied to the design a quasi-passive prosthetic ankle-foot that increases the stiffness with ankle angle using a cam-based transmission and an active sliding support beneath the leaf spring, the strategy cannot be used in a completely passive prosthesis because it requires positive work to be done by the prosthetic, nor should it be necessary for normal walking speeds.
Hansen developed a characteristic of gait called the roll over shape that incorporates both the kinematics and kinetics. The roll over shape is created by plotting the center of pressure during a step in a shank-based coordinate system. Recent research, summarized by Hansen and Childress, has found that “roll-over shapes in able-bodied subjects do not change appreciably for conditions of level ground walking, including walking at different speeds, while carrying different amounts of weight, while wearing shoes of different heel heights, or when wearing shoes with different rocker radii”. This suggests that able-bodied individuals will alter their ankle kinematics to preserve their roll-over shape. However, amputees do not have the adaptive control that an able-bodied individual has over their roll-over shape. Therefore, the design of the prosthetic predominantly controls the roll-over shape an amputee will produce. As a result, it has become a method to evaluate prosthetics. However, while the roll over shape demonstrates the relationship between kinematics and kinetics, it is not directly impacted by magnitude. Other evaluation methods are necessary to determine the late stance push-off.